At present in the Long Term Evolution (LTE) standard of the 3rd Generation Partnership Project (3GPP), peak data rates of a cell have been significantly improved, but rates at the edge of the cell remain far below the peak rates of the cell, and in view of this, numerous studies have been made on improving the throughput of a User Equipment (UE) at the edge of the cell and the average throughput of the cell.
In the LTE system, a relatively narrow Half Power Beam Width (HPBW) of a traditional antenna array in the vertical direction has a uniform down-inclination angle (that is, a uniform beam is provided vertically for each UE in the cell) so that it is very difficult to perform beam scheduling and interference coordination vertically between adjacent cells. The downlink angle of the antennas can be adjusted to thereby improve the performance of the system to some extent, but the downlink angle has to be adjusted very slowly as a transition to 3-dimension (3D) Beam-Forming (BM).
With 3D beam-forming, a narrow beam with a different down-inclination angle can be generated for each UE according to the location of the UE for the purpose of both horizontal and vertical beam-forming to thereby address the drawback of the traditional antennas thoroughly so as to improve the signal to noise ratio of the target UE, thus improving greatly the performance of the cellular system. At present, active antennas controllable per row and/or column have emerged in the industry; and the traditional 2D antennas are provided with only horizontal weighted ports but without vertical ports, and vertical control ports of the antennas can be added to the active antenna system to thereby accommodate a need of vertical beam-forming so as to provide a requisite hardware support for studies on 3D beam-forming.
Channel state information needs to be fed back in order to support transmission of 3D beam-forming, e.g., Channel Quality Indicator (CQI) information, Pre-coding Matrix Indicator (PMI) information and Rank Indication (RI) information, where CQI information is configured for UE scheduling, adjustment to a Modulation and Coding Scheme (MCS) and/or Multi-User Multiple Input Multiple Output (MIMO) pairing, etc., the PMI information is configured for determining beam-forming, multiple-user scheduling, MU-MIMO pairing, etc., and the RI information can be configured for determining the number of layers used for data transmission, etc.
All the channel state information needs to be calculated from channel estimation for which a corresponding Reference Signal (RS) needs to be further obtained. The reference signal, also referred to as a pilot signal, is a known signal provided by a transmitter to a receiver for channel estimation or channel probing. Reference signals for channel estimation in the existing LTE system include a Cell-specific Reference Signal (CRS) and a Channel State Information-Reference Signal (CSI-RS), where the CRS, also referred to as a downlink common reference signal or a cell common pilot, can be transmitted in each downlink sub-frame.
FIG. 1 illustrates a schematic diagram of CRS mapping in the existing conventional Circular Prefix (CP) patterns where each downlink sub-frame is configured with CRS's. R0, R1, R2 and R3 in FIG. 1 represent CRS's configured for antenna ports 0, 1, 2 and 3 respectively, where (a) illustrates a schematic diagram of a corresponding CRS configuration pattern for only one antenna port 0; (b) and (c) illustrate a schematic diagram of corresponding CRS configuration patterns for two antenna ports 0 and 1; (d), (e), (f) and (g) illustrate a schematic diagram of corresponding CRS configuration patterns for four antenna ports 0, 1, 2 and 3. For each sub-diagram in FIG. 1, the y-axis represents the frequency with each box represents a Resource Element (RE); and the x-axis represents a sub-frame including two timeslots (an odd timeslot and an even timeslot), each of which further includes seven symbols (1=0 to 6).
The CSI-RS is a reference signal defined in the LTE system release 10 (Rel-10) as a periodically configured downlink pilot, the CSI-RS is defined in the standard to be transmitted via the antenna ports 15 to 22, and numerous CSI configuration patterns are defined in the existing standard; and FIG. 2 illustrates a schematic diagram of CSI-RS mapping in the CSI configuration 0 in the existing CP patterns, where R15 to R22 in FIG. 2 represent CSI-RS's configured respectively for the ports 15 to 22. CSI-RS sub-frame configurations are as depicted in Table 1.
TABLE 1CSI-RS sub-frameCSI-RS sub-frameCSI-RS periodicalcompensation ΔCSI-RSconfiguration ICSI-RSTCSI-RS (sub-frames)(sub-frames)0~45ICSI-RS 5~1410ICSI-RS − 515~3420ICSI-RS − 1535~7440ICSI-RS − 35 75~15480ICSI-RS − 75
The receiver needs to perform channel estimation on a horizontal and a vertical channel based upon the transmission characteristic of 3D beam-forming to thereby calculate and feed back PMI information corresponding to the horizontal channel and the vertical channel respectively to the transmitter for further 3D beam-forming. The receiver needs to perform channel estimation with knowledge of the configuration of the reference signal, i.e., the configuration of the pilot information. However the existing configuration of the reference signal includes only the configuration of the horizontal reference signal and thus can only support estimation of the horizontal channel but can not support estimation of the vertical channel, thus failing to support 3D beam-forming.
In summary the existing configuration of the reference signal includes only the configuration of the horizontal reference signal and thus can only support estimation of the horizontal channel but can not support estimation of the vertical channel so that information about the vertical channel and a support of 3D beam-forming will be unavailable.